Microsurgical instruments typically are used by surgeons for removal of tissue from delicate and restricted spaces in the human body, particularly in surgery on the eye, and more particularly in procedures for removal of the crystallin lens or the vitreous body. Such instruments include a control console and a surgical handpiece with which the surgeon dissects and removes the tissue. The handpiece has a surgical tool such as an ultrasonic microsurgical cutter for cutting or fragmenting the tissue and is connected to the control console by a long power cable and by long conduits or flexible tubes for supplying an irrigation fluid to the surgical site and for withdrawing or aspirating fluid and fragmented tissue from the site. The cutting, irrigation and aspiration functions of the handpiece are controlled by the remote control console that not only provides power for the surgical cutter (e.g., an ultrasonically vibrated needle), but also controls the flow of irrigation fluid and a provides a source of reduced pressure (relative to atmosphere) for the aspiration of fluid and fragmented tissue. The functions of the console are controlled manually by the surgeon, usually by means of a foot-operated switch.
The multiple connections that are required between the handpiece and the console for the power cable and the suction and irrigation lines have made the preparation and interconnection of the equipment prior to the surgical procedure extremely complex, with the resultant concerns over maintaining the sterility of the equipment and assuring error-free connection. Accordingly, in typical microsurgical instruments, the fluid handling connections have come to be centralized in a "cassette" that contains in one unit all of the connections for the aspiration and irrigation lines, internal conduits for directing the flow of fluids, valves for controlling the flow of fluids into and out of the handpiece, a receptacle for aspirated fluid and tissue and may contain the tube portion of a peristaltic pump. The cassette typically is supplied in a sterile package with color-coded connecting tubing already attached. Thus, setting up the equipment requires only connecting the cassette tubing to the surgical handpiece and irrigation fluid source and inserting the cassette into a receptacle in the console. The receptacle may contain the roller head portion of a peristaltic pump (or some other access to reduced pressure), an aspiration line pressure sensor and devices for operating the valves in the cassette and for controlling the flow of irrigation or aspiration fluids through the fluid conduits within the cassette. For convenience and to maintain sterility, the cassette may be discarded after a single use or sterilized and reused.
Such a cassette is disclosed, for example in Steppe, et al., U.S. Pat. No. 4,713,051. The Steppe, et al., cassette is intended to cooperate with a control console that has the roller head element of a peristaltic pump as well as protruding occluder bars that can pinch or block the internal flexible tube conduits that carry fluids through the cassette. The cassette also has an irrigation transfer tube that vents irrigation fluid into the aspiration conduit to relieve quickly the reduced pressure created within the aspiration conduit of the cassette when the aspiration function is discontinued by the surgeon. When the cassette is installed in the console, an arcuate cutout on the top of the cassette containing the compressible tube portion of the peristaltic pump engages the roller head to supply a source of reduced pressure for the handpiece. The output of the peristaltic pump is collected in a flexible bag suspended from the cassette.
While the Steppe, et al., cassette has proved useful, the use of flexible tubes as internal fluid conduits requires accurate assembly of the tubes within the two-part housing of the cassette for proper operation, a complex, multi-step manufacturing process. Furthermore, the method of occluding fluid flow by pinching the flexible tubes relies entirely on the resilient properties and the quality of the tubes, variables that are not easily controlled during manufacture to assure consistent, predictable performance.
Another microsurgical cassette is disclosed in Sundblom, et al., U.S. Pat. No. 4,758,238. The Sundblom, et al., cassette uses channels molded into the body of the cassette and sealed by a stretchable covering gasket held taut against the molded body by a cover plate as internal fluid conduits. At certain points along the conduits, enlarged round chambers are formed in the conduits that allow enough room for the conduit to be blocked by valve stems stretching the gasket tightly against the conduit inlet to seal the conduit.
The Sundblom, et al., cassette requires not only the use of mechanical valves stems, but also that the gasket perform the dual function of serving as one wall of an extensive system of fluid conduits within the cassette and as a stretchable diaphragm to block the fluid flow at the selected locations. This dual function requires expensive gasket materials (i.e. silicone rubber), a multi-piece cassette and demanding assembly procedures.
DeMeo, et al., U.S. Pat. No. 4,798,580, discloses a microsurgical cassette having a peristaltic pump tube within the cassette that cooperates with a roller drive on the console to provide a source of reduced pressure for the aspiration function of the microsurgical handpiece. The DeMeo, et al., cassette has a vent for venting the suction conduit to the atmosphere when the peristaltic pump is stopped. This vent consists of a flexible tube that is compressed and thereby pinched shut by an occluder bar in the console. The occluder bar pinches the tube, thereby closing the vent, whenever the peristaltic pump is operating and the bar is pulled away from the tube, thereby opening the vent, when the peristaltic pump is stopped to vent atmospheric air into the suction line. The DeMeo, et al., cassette has an irrigation fluid conduit consisting of an inlet port for connection to a source of irrigation fluid, an outlet port for connection to the tubing supplying irrigation fluid to the microsurgical handpiece and a flexible tube within the cassette connecting the inlet port and the outlet port. A portion of the tube can be alternatively pinched shut or opened to prevent or permit the flow of irrigation fluid by pressure from an occluder bar similar to the vent line occluder bar. The DeMeo, et al., cassette, by using compressible tubing as the control valves for the vent and irrigation fluid conduits and a two-part housing, has the same limitations as the Steppe, et al. cassette discussed above.
Accordingly, a need has continued to exist for a surgical cassette that is capable of simple and easy assembly while allowing the selection of reliable, predictable materials and structures for each function performed by the cassette at the lowest possible cost.